An expected three-week-long hearing kicking off Monday in Albany could radically change how the Indian Point nuclear power plant uses water from the Hudson River.

For more than a decade, plant owner Entergy and the state — joined by the environmental group Riverkeeper — have been battling about the most efficient and cost-effective way to do that.

At stake are the lives of more than a billion fish each year. Entergy opposes what the state says is the best way to avoid those deaths.

The state in 2003 said Entergy must stop siphoning billions of gallons of water out of the Hudson each day to cool its nuclear-powered electricity generators because it kills too many fish. Entergy could almost eliminate those fish deaths if it converted to a closed system of cooling towers instead of the current open-loop system, the state argues.

The hearing will focus on whether such an arrangement can be built and operated at the Buchanan plant.

"The conversion of the existing once-through (system) to a closed-cycle cooling system would protect over 1.1 billion fish annually. This includes fish of all life stages from egg through adult," state biologist Chuck Nieder wrote in pre-filed testimony in late February.

Fish die at Indian Point in essentially two ways: larger-sized ones are sometimes trapped on bars blocking debris from the cooling water intakes and smaller fish, larvae and eggs can get sucked into the plant itself. Rotating screens return some fish to the river.

"At this crucial stage in the Indian Point hearings, it's clear from expert testimony and reports in the record that a closed-cycle cooling retrofit of Indian Point would virtually eliminate the plant's impact on the Hudson, and can be accomplished without having any impact on energy reliability for New York City and the Hudson Valley," Riverkeeper attorney Phillip Musegaas said.

A closed system would use much less water and could reduce fish kills by more than 98 percent, the state Department of Environmental Conservation calculated. But Entergy maintains such a system is too big to build at Indian Point.

"Entergy intends to establish that cooling towers are not an available technology for Indian Point due to engineering feasibility, construction and blasting impacts, local zoning laws, potential air emissions impacts, and relocation of major natural gas pipeline," company spokesman Jerry Nappi said.

The utility company has proposed installing "wedge-wire screens." The large, cigar-shaped units would sit on the riverbed and are filled with tiny, vertical slots — allowing water to be drawn in while keeping out more aquatic life. Estimates in the reduction of fish kills range from 51 to 91 percent, compared to the current system. Almost no fish are trapped on the wedge-wire screens and they perform almost as well as the closed cooling towers in preventing fish from being sucked into the plant, Nappi said. The plant would use the same amount of water.

"When you factor in the negative consequences of cooling towers ... it is evident that wedge-wire screens make more sense at this location," Nappi said.

If built, each tower would be 91 feet tall, about half as tall as an earlier plan.

Construction and annual maintenance for 20 years would cost about $1.6 billion, according to the state's evaluation.

In contrast, the new screen system would cost about $200 million to build, according to Entergy.

Entergy says there is no evidence Indian Point harms adult fish populations and argues most larvae never grow up, succumbing to predators, habitat problems and other factors.

Both Riverkeeper and the DEC reject that reasoning. Nieder, the state biologist, said entrapment in a power plant is not "a legitimate use of Hudson River fisheries resources."